1. Field of the Invention
The present invention relates to a seed for cultivation of a new variety of Stevia rebaudiana Bertoni capable of growing leaves containing Rebaudioside A at a high proportion, a new variety of Stevia rebaudiana Bertoni capable of being cultivated by seed propagation and also a sweetening produced therefrom and a process for producing the sweetening.
2. Description of the Prior Art
Stevia is a perennial plant of compositae, of which the provenance is Paraguay and the scientific name is Stevia rebaudiana Bertoni. Stevia contains sweetening components of about 300 times the sweetness of sugar and therefore is cultivated for use in sweetening ingredients as a natural sweetening.
As the sweetening components of Stevia, Stevioside (C.sub.38 H.sub.60 O.sub.18, molecular weight of 804), Rebaudioside A (C.sub.44 H.sub.70 O.sub.23, molecular weight of 966), Rebaudioside C, D, E and Dulcoside A, etc are known.
In the varieties of Stevia generally cultivated, a major component of the sweetening components is Stevioside (ST), the content of Rebaudioside A is about 3 to 4 tenths and that of Rebaudioside C is a little less. However the components of Stevia depend on the variety, for example, one variety does not contain Rebaudioside A or C, and in another variety, the major component is Rebaudioside C.
Sweetness is a particularly delicate sense among the senses which are perceived by the tongue, for example, when compared with astringent or pungent taste. The degree of the sweetness of Stevioside is about 300 times as sweet as sugar and therefore is used as a sweetening in the food industry. The sweetness of Stevioside is relatively similar to that of sugar, but has a disadvantage in that it leaves some unpleasant tastes such as a bitter taste. In other words, it is undesirable for a sweetening to contain large amounts of Stevioside. On the contrary, Rebaudioside A has a good quality sweetness and the degree of sweetness is 1.3 to 1.5 times as sweet as Stevioside.
The inventors of the present invention carried out plant breeding by the repetition of crossing selection to obtain varieties of Stevia which show a high content ratio of Rebaudioside A (RA) in relation to Stevioside (ST). They have produced an excellent sweetening by extracting sweetening components from a variety of Stevia (Japanese Patent Publication A Nos. 59-045848 (1984) and 61-202667 (1986)).
However, the content proportion of Rebaudioside A in the above mentioned varieties decreases in plants cultivated by seed propagation, for example, a plant from a seed in a ratio of ST:RA of 1:4 grows leaves in a ratio of ST:RA of 1:1 to 1.5. Therefore, in order to keep a high content proportion of Rebaudioside A, cuttage has been used.
In addition, Stevia rebaudiana Bertoni is self-incompatibile and therefore, the harvested seeds do not always germ. That is, the germination percentage is low. That is the reason why Stevia has been cultivated by cuttage and those skilled in the art would not consider cultivating Stevia by seed propagation.
However, since the number of cuttings obtained from a parent plant are limited and many parent plants are needed, it is difficult to cultivate a large quantity of rooted cuttings.
The inventors intensively studied Stevia capable of being cultivated by seed propagation while maintaining a high content of Rebaudioside A.